Russian President Vladimir Putin has called for an immediate end to what he called the Ukrainian government's "punitive" military operation in the east, after dozens were reported killed in a day of fighting in the eastern city of Donetsk.

Ukraine's Interior Minister Arsen Avakov said on May 27 that government forces are now in control of Donetsk airport, which separatists tried to seize the day before.

The Kremlin said Putin had spoken to Italian Prime Minister Matteo Renzi by telephone and "underscored the need for an immediate halt to the punitive military operation in the southeastern regions and the establishment of peaceful dialogue between Kyiv and representatives of the regions."

There is no official death toll from the fighting. Separatists said at least 30 and possibly up to 100 fighters were killed.

The mayor of Donetsk said 40 people were killed, including two civilians and 38 "combatants."

It was the first time the fighting has spread to the city of Donetsk since the unrest in eastern Ukraine began some two months ago.

A group of separatists on May 27 set fire to part of an ice-hockey arena in Donetsk, but firefighters have since brought the blaze under control. No injuries were reported.

Meanwhile, the special observer mission of the Organization for Security and Cooperation in Europe (OSCE) in Ukraine says it has lost contact with one of its teams in the Donetsk region.

The OSCE said in a statement on May 27 that the last time it had been in touch with the four-member team was on the evening of May 26, when the monitors were on a routine patrol east of the city of Donetsk.

An OSCE spokesman said that the monitors, all men, were Turkish, Swiss, Estonian, and Danish.

A Danish government minister said that the missing monitors were believed detained by pro-Russian separatists in Ukraine.

There was no immediate claim of responsibility, but rebel groups have previously kidnapped OSCE monitors in Ukraine.

INTERACTIVE MAP: Fighting in eastern Ukraine (mouse over for more info):

In the neighboring Luhansk region, Ukraine's Border Guards Service said border guards opened fire overnight on May 27 on a convoy of vehicles carrying rebels that had allegedly crossed over from Russia.

At least one rebel was killed in the firefight. The border guards also said they seized several vehicles allegedly carrying weapons.

First Deputy Prime Minister Vitaliy Yarema said that the "antiterrorist operation" will continue "until not a single terrorist remains on the territory of Ukraine."

Petro Poroshenko, the winner of Ukraine's May 25 presidential election, said he wants talks with Moscow to resolve the crisis, but Russian Foreign Minister Sergei Lavrov said on May 27 that a visit to Moscow by Poroshenko is "not being considered."